How to make a texture that shines, or paint a material?
You UV unwrap and texture your mesh with a glossy map. The parts you paint white will be glossy, and the parts left black will be diffuse. Use that glossy map to factor a Mix Shader that has a Diffuse Shader and a Glossy Shader connected to it.



If your Glossy and Diffuse shaders are plugged in the opposite way then it will be inverted, with black values being glossy and white diffuse, so pay attention to how you've set up your mix node. Any areas that are grey will be neither fully glossy nor fully diffuse, but something in between.

Of course this concept can be applied to the factoring of any mix, whether it be glossy/diffuse, red/blue, bumpy/smooth, transparent/opaque... the possibilities are infinite.